The present invention relates to a method and a device for controlling terminal voltage differences between individual battery cells in a battery during the charging thereof, wherein the terminal voltage is measured for each individual battery cell, differences between the terminal voltages are determined, and a portion of the charging current is shunted in parallel to at least one of the battery cells in response to the determined terminal voltage differences.
Rechargeable batteries are used in many different portable applications, such as cellular telephones, communication radios, portable personal computers, video cameras, etc.
In these applications a rechargeable battery comprises a plurality (preferably 2-5) of battery cells, which are connected in series with each other in order to provide, when viewed as a unit, the desired supply voltage of between, for instance, 3 and 6 volt. Conventionally, battery cells of NiCd-type (nickel-cadmium) have been common, but today NiMH-type cells (nickel-metal hydride) as well as Li-type cells (lithium-ion, lithium-polymer) are more common.
It is desired, when charging a battery comprising a plurality of cells, that all cells contained in the battery exhibit essentially the same terminal voltage. In practice, however, there will often be a lack of balance between the cells during the charging, so that one cell will become fully charged after some time, while another cell will still need some time before it becomes fully charged. Such unbalance is undesired from a performance point of view as well as for safety reasons.
In the Swedish patent publication SE 451 924 a regulator is shown for controlling a charging current to an individual cell in a battery of cells. The regulator takes care of the problem of different cells becoming selfdischarged after different periods of time by shunting, by means of a shunt circuit, a portion of the maintenance charging current to the individual cell in response to the difference between the terminal voltage of the cell and a reference voltage. According to this solution the regulator will not become active until at least one battery cell has reached a state of final charging voltage and the battery has entered a state of maintenance charging. The disadvantage of having to postpone the shunting of the charging current, or a portion thereof, until one battery cell has become fully charged, is that the shunted current has a high amperage, which will cause major losses in power due to the generation of heat in the battery.
WO95/15023 relates to a dynamic balancing and monitoring device for an accumulator battery pack. The charging characteristics, or the terminal voltage, of each battery cell is monitored, and if the charging characteristics of a particular battery cell starts to deviate from the rest of the battery cells or from a normal condition, a portion of the charging current to that battery cell is shunted in parallel to the cell. Thus, the charging conditions between the battery cells are automatically balanced or equalized.
EP-0 662 744 relates to a device for balancing the charging conditions between series-connected battery cell pairs. U.S. Pat. No. 5,504,415 relates to a method and an apparatus for equalizing the state of charge among a plurality of series-connected batteries of a similar kind.